


Ties that Bind

by OopsIShipTheThingArchive



Category: Big Hero 6 (2014)
Genre: Alpha Tadashi, Alpha/Beta/Omega Dynamics, M/M, Omega Hiro
Language: English
Status: In-Progress
Published: 2015-02-19
Updated: 2015-02-25
Packaged: 2018-03-13 18:20:29
Rating: Explicit
Warnings: Underage
Chapters: 2
Words: 4,528
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/3391526
Author URL: https://archiveofourown.org/users/OopsIShipTheThingArchive/pseuds/OopsIShipTheThingArchive
Summary: <blockquote class="userstuff">
              <p>Presenting as an alpha or omega is always stressful.  But when you're pretty sure you're supposed to mate with your brother?  That adds a whole new level of complicated.</p>
            </blockquote>





	1. Chapter 1

**Author's Note:**

> Yooo I wrote my first a/b/o fic! Rating because of smut in the next chapter. I have a few notes about my personal interpretation of a/b/o dynamics at the end of the chapter. Also you will notice the lack of mpreg tag. Yes it is going to stay that way, not my cup of tea.

Everyone was surprised when Tadashi presented.  At twenty-one years old he was older than average for it, but not into the realm of the unusual. However, quiet and mild-mannered as he was, everyone had assumed he would be a beta, or maybe even an omega. And that was why GoGo had almost jumped him in the lab when he got too close to Honey.  This wasn’t the Middle Ages when such instinctual displays of dominance were acceptable, but GoGo had been caught completely by surprise. Honey was showing off her latest compound (which was apparently very structurally interesting but all GoGo saw about it was its shockingly pink color).  Tadashi leaned over to check it out, inhaled the fumes by accident, and shook his head before sneezing hard.

The bitter-acrid scent of another alpha hit her like a wall of bricks.  Luckily her snarl of challenge gave just enough warning, and in seconds Honey and Wasabi had lifted her off the floor and borne her away from the direction that Fred was ushering a thoroughly confused Tadashi. “How would you feel if you were standing next to your mate and then got hit with a face full of unbonded alpha pheromones out of nowhere?” (She asked Tadashi over the phone that night, thoroughly disgruntled with the whole affair,) “I mean, I’m sorry I tried to attack you but you couldn’t have given me a little bit of warning? ‘Oh, GoGo, I might present as an alpha today right next to your girlfriend but I don’t swing that way so please don’t try to bite me’?”

“You were going to bite me?” Tadashi sounded amused and a little intimidated.

“Only if I had to, bonehead,” GoGo huffed.  “And congrats on the alpha thing, I guess.  Good luck.”

Good luck. He’d never needed luck so much in his life.  Aunt Cass bubbled happily about how many cute omegas were going to notice him now (“How would you know? You’re a beta.” “Oh, guardians just know these things I guess!”) and then treated him to an absolutely mortifying lecture about when it was and wasn’t acceptable to breed an omega in heat.

Hiro didn’t say anything until Tadashi had collapsed into bed that night, exhausted by the drama and physical demands of the day.  Then he padded swiftly over, hopped up into bed with him and stuck his nose into the fine hairs at the back of his neck.  Tadashi shivered at the attention, furiously forcing himself to think about the slight itch of his pillowcase against his cheek rather than anything Hiro was doing as the boy sniffed him thoroughly.

“Dunno what the big deal is, you still smell the same to me.” He grumbled, vaulting off the mattress (and yanking Tadashi’s blanket off in the process).  Tadashi groaned into his pillow, hoping that it would all go away in the morning.

***

The morning made it all approximately one hundred million times worse.  He woke to the crawling feeling that his skin was two sizes too small. He was sweating and overheated and _he’d never been this horny in his life._ Thank god Hiro was an early riser and wasn’t in the room to listen to his brother frantically masturbating once, twice, three times.  Cass finally came up to check on him around noon, finding him a quivering lump under the blankets.  She didn’t stay long, just picking up Hiro’s dirty clothes from the night before, then wandering over to lay a gentle hand on the quilt.  “Let me know if I need to keep Hiro downstairs tonight.  And wash those sheets when it’s done, alright?”

Tadashi stuck his head out from under the fabric, red and shame-faced.  “I’ll be okay by tonight.  It’s already going away.  Sorry Aunt Cass. And please don’t say anything to Hiro?”

If Aunt Cass thought it was an odd request she didn’t say anything.  By the time Hiro returned from wherever he went that night Tadashi’s bed sheets were freshly laundered and Tadashi had almost completely regained his dignity. As far as he knew, Hiro never found out that Tadashi had his first rut that day, or that it had been triggered by a dream about his baby brother.

***

“You’re back after _two days_?” GoGo demanded.  “You took _two days_ to get over presenting?  I hate you!”

Tadashi held his hands up defensively. “It wasn’t that bad! I mean, the…um…the first bit was…weird.”

“Yeah going in to rut for no reason’s pretty annoying,” GoGo agreed, looking up at him with narrowed eyes, not missing the way he blushed at that.  “Not to mention my first one lasted for almost a week. And all the smells! How are you dealing with those?”

Tadashi could only shrug helplessly.  He’d known that for a rut his had been easy.  Frequently they lasted for days, and occasionally newly-presented alphas would even end up arrested for getting into fights on the street, or attempting to force themselves on an unwilling omega.  He’d been, well, distinctly uncomfortable, and it was probably a good thing there wasn’t anyone else presented in the household, but he hadn’t been driven out into the street to find an outlet for his aggression.

As for the smells, he’d come down to the café the second day and almost had to crawl back up the stairs. The mingle of sweaty bodies was already completely overwhelming, but adding in the scent of alphas and omegas had his eyes watering.  Alphas reeked of something like dark coffee, a bitter burnt smell that stung his nose. Most omegas took heat suppressants nowadays but they still smelled enticing, a unique flavor that left a sweet taste on the back of his tongue, though most of the individual scents were cloying and slightly overwhelming.

He’d ventured back downstairs a few hours later, following a particular smell that drifted up throughout the apartment.  At first he’d thought his aunt was baking butter cookies, but something wasn’t quite right. He’d tracked the scent in and out of the kitchen in the back and the café in the front, then back up the stairs to his room.

That was part of why he was back at school already, because he felt like he couldn’t trust himself around his brother.

***

When Hiro presented no one was surprised.  It would be more accurate to say that everyone was shocked.

Everyone expected him to be an alpha.  A surly, difficult child who had grown into a rebellious teenager, of course everyone expected him to be an alpha.  But given that it was almost unheard of for a child to present before the age of sixteen Aunt Cass figured she had a few years to figure out how to deal with two hormonal young alphas sharing a room.

So it was a surprise when, just two weeks after Tadashi presented, she was awoken by pounding feet on the stairs and Tadashi’s panicked calls.  “Something’s wrong!  Something’s really wrong with Hiro!”

Cass lurched out of bed and was heading up the stairs before she was fully awake, and was almost run over by Tadashi on his way down.  His eyes were wide and dark in his pale face, and he looked like he might be sick. Given that, Cass was fairly certain what she would find in the attic bedroom.

She took a moment before rounding the corner to take a deep breath, bracing herself to face an out-of-control young alpha in the grip of his first rut, and to restrain him if she needed to. Instead, she turned the last corner to find Hiro on the floor next to his bed, curled up and clutching his lower stomach with weak little whimpers.

“Oh, Hiro.” He didn’t often get sick but when he did it was always spectacular, frequently requiring doctor visits, and a few times hospitalization.  She sighed, shoulders dropping as she relaxed, and crossed the messy floor to his side. “Is there anything you need, kiddo?”

He shook his head, hard and frantic, his eyes clenched shut.  It was only then that she spotted the sweat beading his forehead, the way he panted lightly through his open mouth, and the way he had shifted to hide his lower body from her sight.  “Oh honey.”

The panting slowly formed into words and she leaned forward until she could hear him.  “Hurts, burns, hurts, burns, hurts…”

“It’s a heat, honey, I know it hurts.” She gently brushed sweat-sticky bangs off his forehead.  Poor kid was clammy and damp all over, even as he panted as if he was burning up. “Come on, let’s get you back into bed and I’ll get you an icepack.”

She managed to haul him up onto his mattress, a process that was complicated by the way he was determined to keep his hands or arms covering his crotch, and when she released him he rolled onto his front with a miserable little moan.  “I’ll be right back,” she called, and received no answer. He could probably use some painkillers too.

She returned to find him on his side again, curled into a fetal position while he panted harshly. “Here.” She held out an icepack wrapped in a washcloth.  “You can put that wherever you feel too hot.  I brought you some advil if you want it.”

He nodded wordlessly, sitting up just enough to sip from the cup of water she held out for him and to swallow the two pills.  He reached out for the icepack and pressed it to his lower abdomen with a wince.

Cass waited for him to settle again, then reached out to feel his forehead.  He wasn’t actually running a fever, which was good. The heat didn’t actually raise the omega’s body temperature, just created a hot sensation because of increased blood flow to certain areas.

Speaking of which—“You know what you can do to make this better, right?” she asked, trying to be tactful.

Hiro nodded again, blushing up to the tips of his ears and refusing to meet her gaze.  “Alright, I’m going to leave you alone,” she said, kissing his forehead before she stood up.  “I’ll be downstairs if you need anything.”

He stayed silent until she reached the door, then “Aunt Cass?  This means I’m an omega, right?”

“Yes, it does,” Cass said, pausing to see if there was anything else he needed.

“Everything’s going to change now, isn’t it?”   Hiro sounded absolutely miserable, and she turned to see wide brown eyes fixed on the floor.

“Of course not, sweetie. You’ll have to stay in for this first heat and then we’ll get you to the doctor for some heat suppressants and then everything will go back to normal.”

Hiro didn’t look like he believed her.   “Can Tadashi come sit with me?” he asked plaintively.  “I want Tadashi.”

“I don’t know if that’s a good idea,” Cass said slowly.  There was no way Tadashi would lay a hand on his brother, not under normal circumstances, but then he’d only presented two weeks ago.  He almost certainly hadn’t encountered an omega in heat since then and probably had no idea how powerful the urges could be. Not to mention the chance that he might be sent into a rut by the presence of an omega in heat in his own room.

Hiro looked down and away, brows furrowing.  “I know. But I want Tadashi. Please?”

Really, she shouldn’t. Even she could smell him, though to her the odor was faint and not particularly enticing.  But there was no way Tadashi would be tempted, right? They were brothers.

“I’ll ask him if he’s up to it, alright?” she gave in grudgingly.  “And I’m going to supervise you two for a while.”

Hiro scowled at her, though whatever he was going to say was interrupted when his face contorted with pain and he clutched at his stomach again.  “Just go away and get Tadashi,” he muttered, voice strained.

Cass sighed and trotted down the stairs.  She hoped that Tadashi had left the house and she wouldn’t have to pass on Hiro’s message, but no such luck.  The man was anxiously pacing the living room, his hands running nervously through his hair. “It’s a heat, isn’t it?” he asked as soon as he spotted her, and she nodded.

“He wanted to know if you’d sit with him,” she said carefully.

Tadashi looked like he was giving the idea serious thought, at least.  “Yeah, I can do that.”

“I’m coming up with you then, at least for a while.”

He looked shocked. “Aunt Cass!  He’s my brother!  I’d never—“

“Alright, alright! But biology has made people do weirder things before.”  She held up her hands and led the way up the stairs again.

She didn’t miss the way he paused on the landing, nostrils flaring and hands clenching at his sides, but by the time they reached the boys’ shared room there was no sign of tension in his face or body, and she had to step hurriedly out of his way as he skidded over to kneel next to Hiro’s bed.

The teenager had rolled over onto his other side, his back to the door, but when Tadashi threaded his fingers through his wild hair Hiro rolled over and made a pathetically sad face up at him, and Cass had to smother a laugh.

“Hey, knucklehead, how’s it going?” Tadashi asked, and Hiro scowled, his expression saying more than words could.

“How do you _think_ it’s going?  If you’re just going to ask stupid questions you could go out and get me beef jerky or something.”

“Beef jerky?” Tadashi asked, amused.

Hiro flopped one limp arm over his face.  “I just really really want it right now and I don’t know why.  I don’t even _like_ beef jerky. Come on, hand me my notebooks, I might as well get something done while I’m stuck in bed.”

Fifteen minutes later Cass left them alone, shaking her head.  From the looks of things, all she had to worry about was Hiro throwing pencils at Tadashi’s head if he didn’t like his older brother’s ideas. That was a relief, she hadn’t liked the thought of splitting them up at all.


	2. Chapter 2

**Notes for the Chapter:**

> Soooo the planned smut happened and got longer than expected so I'm going to put the estimated length of this fic at four chapters instead of three whoops.

Hiro was in _agony_.

He’d spent the day in bed, getting up only to retrieve more painkillers or a new icepack. Tadashi had been wonderfully understanding as his little brother shooed him out of the room, then called him back in, only to repeat the process all over again.

He wanted, _needed_ Tadashi with him, but at the same time his brother smelled so _good._ The scent was so strong that he could taste it on the roof of his mouth, something dark and wild and he wanted Tadashi to pin him down and—

No.  No no no no _no._ He was not a slave to his instincts or his desires and he was not going to try to entice his _brother_.  Or, as his aching burning body demanded, kneel and present himself for Tadashi. Whenever it became too much he would send Tadashi on a random errand, or (depending on how bad the pain was at that point) snap at him to go away.  He used some of these moments of respite to scamper into their connected bathroom, wiping away the slick that had dampened his boxers and skin before he pulled his pants back up.

Oh he had tried touching himself, but all that achieved was over-sensitized burning and another distressing trickle of wetness between his legs.  He’d even tried to work his own fingers inside himself, trying not to think about what he was doing as he contorted himself into weird positions and soaked his hand with his juices and was reduced to frustrated sobs because it _wasn’t quite enough._

By bedtime that night the heat had only gotten worse, and at that particular moment Hiro had cocooned himself in his blankets and resolutely faced away from his brother. His stupid, perfect brother who kept asking him if he needed anything!  He was tempted to answer “yes, your dick” just to see the look on Tadashi’s face, but it was also far too close to the truth for comfort.

So instead he suffered in silence, trying not to whimper with each wave of aching heat that passed through his lower body, and will himself to sleep anyway.  He thought he was succeeding until suddenly there was another warm body in the bed behind him, Tadashi’s scent filling his senses as his older brother scooped him up in his arms and rubbed his nose on the back of Hiro’s neck.

“W-what are you doing?” Hiro stuttered, feeling himself blush when he moved his legs and felt the distinct slide between his thighs that meant he was leaking again.

Tadashi pulled him closer, despite his squirming, tucking his nose behind Hiro’s ear.  “I can hear you rolling around from all the way over there. When you were little it always helped you sleep if I came and slept in your bed, so I thought I’d try it tonight.”

Hiro grumbled but stayed still, worried that if he moved too much Tadashi would notice how wet he was. His boxers were clinging to him stickily and there was still an uncomfortable throbbing behind his pubic bone and Tadashi’s presence was _not helping at all_ thank you very much.  He tried to keep quiet but then Tadashi’s hand was running up over his ribs, and he let out an embarrassing gasping whimpering noise.  Instantly Tadashi froze.

“Are you okay?”

“No, I’m not okay!” Hiro snarled, pain and mortification making him irritable.  “I keep soaking through my underwear, everything in my room smells like alpha, I want to be bred so badly it’s making me dizzy, and I hurt so much that I thought I might puke earlier so I’m not okay and I haven’t been okay all day!”

Tadashi was silent behind him, barely even breathing, and Hiro had several long minutes to thoroughly regret his outburst.  He certainly hadn’t meant to say all of that, especially not the part about wanting to be bred. He sighed and shoved his face into the pillow, waiting for Tadashi to get up and leave, to call him sick and wrong.  But instead—

“You tried touching yourself, right?”

What kind of question was that? Hiro nodded, staring resolutely ahead and pretending Tadashi couldn’t see the way his ears must be reddening. Another long awkward silence, while Tadashi seemed to be waiting for an elaboration on that answer. Hiro was the one who gave up, heaving in a long inhale (which unfortunately nearly overwhelmed him with Tadashi’s scent).  “It didn’t work,” he muttered.

“Maybe you should try again?” Tadashi suggested mildly.  “It’s supposed to help.”

It probably wasn’t what Tadashi meant, but with the scent of alpha everywhere around him and Tadashi’s large hands (had Tadashi’s hands always been that big?) gripping his waist, Hiro wasn’t thinking particularly clearly.  He arched his back and worked his hand down under the waistband of his boxers and ran light fingertips up the underside of his erection.

That was much better, no overstimulation from gripping himself too tightly, and Hiro repeated the motion. On the third time his hips bucked up into the feeling and he bit down on his lip to muffle a moan. Tadashi stiffened behind him, apparently only now realizing what he was doing.  “Do you want me to give you some privacy?”  His voice was choked with some emotion that Hiro couldn’t begin to guess at, especially when he was frantically shaking his head in answer to his brother’s question.

“Don’t.  Please stay,” he gasped.  This felt so _right_ , surrounded by the heat and hard muscles of an alpha (his alpha) and breathing in the intoxicating heady scent with every inhale.  He palmed himself, spreading the precome leaking from the tip of his cock, then reached down further to press his fingers against his wet opening.

Tadashi’s breath was slow steady heat against the back of his neck, and he thought Tadashi’s hips might be moving too but it was hard to tell.  Hiro squirmed and wriggled, bucked and arched, working just the tips of his first two fingers into himself and gasping at the feeling.  He shoved his other hand into his boxers, chasing the feeling that was just out of reach—and let out a sob of frustration at the sickly sting as he over stimulated the raw nerves again.

At the noise Tadashi let out a _growl_ behind him, yanking him close with both arms around his waist and there was a sudden harsh pressure of teeth on the back of his neck.  Rather than being uncomfortable it only served to stoke Hiro’s arousal higher, and he bent his head to the side in an attempt to bare more skin for his brother. He whimpered when Tadashi bit him again, panting openmouthed when there were long fingers working up under his shirt and stroking over the stark lines of his ribs.

“Tadashi, please, I need…need…” Hiro cut off and helplessly ground his hips back against the firm muscle of his brother’s abdomen.

“ _Fuck_ , Hiro—“  He shuddered all over; Tadashi _never_ swore around him, and his tone suggested he was holding himself in control by only the barest margin. Then Tadashi’s hands were sliding lower, slipping under his boxers and squeezing his ass. He spread Hiro’s cheeks apart and traced lightly down the space between them.  He paused right before he touched where Hiro wanted him most, and with a little whimper the omega realized Tadashi was waiting for him to give the okay.

He canted his hips, trying to push down onto those tantalizing fingertips, and when Tadashi moved away he mewled “Please!”

“ _Fuck.”_ Tadashi’s middle finger slid easily inside him, aided by how wet he was, and Hiro let out a strange sound halfway between a moan and a yelp as stars burst behind his closed eyelids. That was _exactly_ what he needed, not anywhere near as thick as a knot but much much better than his own touch.  Tadashi paused with the bulge of his second knuckle stretching Hiro’s rim, then pumped that single finger in and out one slow time.

Hiro was too startled even to cry out as his orgasm swept over him, his body arching and he vaguely registered the impact of the back of his head against Tadashi’s chin as cool relief followed in the wake of the pleasure.  He went limp in his brother’s hold, panting, then groaning as his arousal abated for a few precious seconds.  There was hot rapid breathing against the back of his head and he rolled over on his stomach, twisting his head so he could look up at his brother.

Tadashi gently removed both his hands from Hiro’s clothes and, apparently not realizing that Hiro was looking, he brought his finger up to his mouth and _licked_.

Hiro’s head fell forward with a punched-out moan, going from zero to so-hard-he-ached in the space of a few seconds.  He kept his face pressed into the pillow because he was fairly certain that if Tadashi did something like that again, or worse, looked at him with those hooded lust-darkened eyes, he might just explode.  Or do something infinitely more embarrassing like come without even being touched.

Of course his brother didn’t leave him alone, and as Tadashi’s hand stroked up his spine Hiro wasn’t sure if that was a good or bad thing.  The heat was back, burning just under his tailbone, and with every glide of Tadashi’s fingers over his skin it just got worse.

Oh, fuck it all. Clearly Tadashi was willing to push boundaries tonight and right now Hiro needed an alpha far, far more than he needed his brother.  The omega clenched his hands in the pillowcase, tried not to think about what he was doing, spread his legs, arched his back, and whispered “ _Please_ ‘Dashi.”

Tadashi mumbled something that was probably more expletives but Hiro couldn’t make out exactly what was said because his brother had just tugged his boxers down over his hips and—if that feeling was what he thought it was—was nipping and licking at the dimples of his lower back.  He moaned without restraint, body reacting on instinct as he pushed his ass further into the air for his brother’s mouth and fingers and—

But it didn’t sound like Tadashi was undressing at all.  Instead the elder pressed his forehead to the small of Hiro’s back, and with a gentle push he slid two fingers into the omega and slowly rotated his hand until they brushed against _something_ that sent Hiro lurching forward into the pillow with a gasp.  Somehow the fabric had ended up in his mouth and he clamped his teeth on it, whimpering as Tadashi pressed mercilessly on that one spot.

Then Tadashi shifted, leaning over and that was _definitely_ his tongue lapping at the slick that dripped out around his fingers and Hiro was so close to overwhelmed that all he could do was pant around the obstruction in his mouth and try to last just a little bit longer.

He didn’t make it more than thirty seconds before Tadashi _growled_ behind him, a deep predatory _possessive_ sound and Hiro freed one hand from the pillowcase and reached back to frantically palm his own cock.  Tadashi moaned when he realized what Hiro was doing and then his tongue was working _inside_ the younger between his two long fingers which _still_ hadn’t let up on his prostate and Hiro actually _screamed_ when he came.

Every muscle in his body went watery and he slowly collapsed forward onto the sticky mess he’d made of his sheets. He was vaguely aware of Tadashi withdrawing slowly from his body and tucking a blanket around him before he padded away, and the double surge of oxytocin on top of everything else had him asleep within moments, not allowing him enough time to worry about where Tadashi was going.

***

Hiro woke up in a bed that wasn’t his, clean dry sheets drawn carefully up over his body, his heat still pulsing at the center of his pelvic girdle, and with serious confusion about what had happened last night.  He didn’t waste time wondering if Tadashi had actually done what he thought he had. A pleasant ache around his entrance (providing a counterpoint to the throbbing of his heat) told him that he hadn’t imagined a thing.  Then he’d fallen asleep and…what?

He sat up cautiously, and blinked at the blank walls of the divider that surrounded Tadashi’s bed. He was in Tadashi’s bed? When had that happened?

And then the full implications of what he’d done last night hit him and he folded over with a groan. What was going to happen now? How could he face his brother after that? He _knew_ it was only his heat and neither of them would ever have done that if they were in their right minds and yet.

And yet he couldn’t bring himself to regret what had happened.

**Author's Note:**

> So in my mind in a/b/o verses there is no mpreg. It is true that only alpha/omega pairings can reproduce, but they also have to be male/female pairings. So an alpha female and omega male can make babies, as can an alpha male and omega female. However, same sex couples cannot reproduce. I imagine that in the very very rare instances that a trans male omega carries a pregnancy the media loves to jump all over it (think of the major panic over bird flu or the E.coli "virus") so there's a fairly widespread misconception that male omegas can become pregnant.  
> There are ruts and heats, alphas have knots and the male omega's rectum contains specialized secretory cells like the vagina for lubrication purposes. It is not known what the purpose of this is for "traditional" p in v sex.  
> *graphic science rant end*


End file.
